This invention relates to charged particle beam scanning of a workpiece and, more particularly, to a method of and apparatus for highly efficient, highly uniform ion beam scanning of a workpiece, such as a semicondutor wafer.
Ion implantation has become a standard technique for introducing impurities into semiconductor wafers in a controlled and rapid manner. A beam of ions is generated in a source and directed with varying degrees of acceleration toward the semiconductor wafer. The impurities are introduced into the bulk of semiconductor wafers by using the momentum of the ions as a means of embedding them in the crystalline lattice of the semiconductor material. Uniformity of impurity concentration over the surface of the semiconductor wafer is of utmost importance in semiconductor processing. In addition, one of the major objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time.
One way to achieve high throughput is to simultaneously process a number of wafers in a batch. Such systems typically involve mechanical movement of wafers in relation to a beam which is scanned in one dimension. Batch processing systems, however, are generally large to accommodate the batches and are generally used only for high dose implantations. In addition, throughput is less than optimum because of the time required to manually change batches. Furthermore, if the processing system experiences a problem, large numbers of expensive semiconductor wafers can be destroyed.
Another approach has been to process wafers one at a time and to employ automatic wafer handling to improve throughput. The wafer is typically held stationary and the ion beam is electrostatically scanned in a two dimensional pattern over its surface. Such a pattern is disclosed in U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner. Constant amplitude scanning signals are applied to x and y deflection plates to deflect the ion beam in a square Lissajous pattern. The scanning signals are scaled in amplitude to insure that the square pattern covers the round semiconductor wafer. (Wafers typically have one flat edge, but for present purposes this can be ignored.) In addition, the dimension of the square pattern is made slightly larger than the diameter of the wafer to provide a certain amount of overscan. Overscan is necessary to avoid nonuniformities in the doping of the wafer when the beam reverses direction after each scan line and to allow for variations in wafer diameter and position. Furthermore, as the cross-sectional dimension of the ion beam increases, the amount of overscan must be increased to insure that the beam is entirely off the wafer before it is reversed. It can be seen that time spent by the system in scanning the corners of the square pattern outside the periphery of the wafer is unproductive in terms of ion implantation and reduces system throughput. In typical prior art systems, the time spent by the system in scanning portions of the pattern outside the periphery of the semiconductor wafer has been as much as 30% of the total scanning time.
An arrangement for reducing wasted scanning time and confining the scan pattern to the general shape of a circle is disclosed in U.S. Pat. No. 4,260,897, issued Apr. 7, 1981 to Bakker et al. Semicircular conductive elements placed on opposite sides of the wafer detect the ion beam when it scans off the wafer and cause reversal of the scan direction. However, such an arrangement adds complexity to the system. Furthermore, the detector elements are subject to degradation by the ion beam and must be changed to correspond to the size of the semiconductor wafer being processed.
It is a general object of the present invention to provide a new and improved method of and apparatus for scanning a charged particle beam over a workpiece.
It is another object of the present invention to provide a method of and apparatus for scanning a charged particle beam over a workpiece in a highly efficient pattern.
It is yet another object of the present invention to provide a method of and apparatus for scanning a charged particle beam over a workpiece in a highly uniform pattern.
It is still another object of the present invention to provide a method of and apparatus for scanning a charged particle beam over a workpiece in a pattern which enhances the speed with which workpieces are processed.
It is a further object of the present invention to provide a method of and apparatus for scanning a charged particle beam over a workpiece in a pattern corresponding in shape to the shape of the workpiece.
It is a further object of the present invention to provide a method of and apparatus for scanning a charged particle beam over a workpiece in a pattern which is selectable in size to correspond to the size of the workpiece.